We will study the impulse activity in afferent C fibers from the heart during myocardial ischemia, the effects of activity in both C and myelinated afferents from the heart on efferent activity in cardiac sympathetic and vagal nerve fibers, the inter-relationship of the excitatory spinal cardio-cardiac reflexes with inhibitory vagal medullary reflexes and the role of asymmetric cardiac innervation in responses to myocardial ischemia. The experiments will be done upon both cats and dogs. The techniques involve isolation of nerve fibers, either afferent or efferent, and recording from single units during occlusion of the coronary arteries. Occlusion is produced either by ligature or by cessation of coronary inflow in pump perfused coronary arteries. Data is analyzed for qualitative correlations. Standard electro-neurographic techniques are used for nerve recording and the usual methods of recording blood pressure and blood flow will also be employed. Reversible blockade of the cardiac nerves will be done using thermodes applied to the appropriate nerves.